1. Field of the Invention
The present invention relates to a wired/wireless integrated network operating at several tens of GHz, i.e., a radio-over-fiber (RoF) system, and more particularly, to an apparatus for and a method of generating millimeter waves required for the RoF system.
2. Description of the Related Art
Next-generation wireless communication networks required to satisfy both an increase in the number of communication service types and an improvement in the quality of service should use high carrier frequencies, e.g., millimeter waves, instead of conventionally used frequencies. This means that the establishment of a network having a small cell size is required. In this case, linkage with an optical communication network is essential for the efficient establishment and management of a system. Such a system in which an optical communication network and a wireless communication network are linked is called a radio-over-fiber (RoF) system.
The core portions of a RoF system are a portion for generating millimeter waves and a frequency up-converting portion for up-converting a digital data signal in a low-frequency band to a millimeter-wave band. Recently, extensive research has been conducted into the implementation a millimeter-wave generator and a frequency up-converter using an optical method in order to establish a low-cost system. Millimeter-waves may be generated by locking several light diodes (LDs) or by using a high-speed external modulator.
However, when several LDs are locked, stringent operating conditions between the different LDs, e.g., temperature, injected currents, and polarization states, should be met. In addition, when an external modulator is used, an expensive device is required for generating millimeter waves.
Frequency up-converters have also been extensively researched. As part of this research, frequency up-converting methods which use the non-linearity of a mach-zehnder optical modulator (MZM), a photo diode (PD), and a semiconductor optical amplifier (SOA) have been suggested. In particular, an all-optical remote frequency up-converting method by a SOA has high conversion efficiency by SOA gain, has small variance in the polarization of a signal by the SOA that is independent of the polarization of transverse electric (TE) and transverse magnetic (TM) modes, and can perform up-conversion when operating in a data signal bandwidth. However, the frequency up-converter is implemented as a separate device independent of the millimeter-wave generator.
Thus, conventional methods for implementing millimeter-wave generation and frequency up-conversion by two independent devices are not cost-effective. Although research into the implementation of these two functions using a single device has been conducted for the establishment of a low-cost RoF system, efforts are still ongoing in cost and technical terms.